The high frequency characteristics of a MOSFET have improved in connection with microfabrication process of the CMOS in recent years, and consequently a high frequency amplifier can be realized. In the high frequency amplifier, it is important to match the impedance of input and output in a desired band and to stable a circuit outside the desired band.
FIG. 11 is a circuit diagram of a source grounded FET type amplifier. A signal input from an input terminal 12 passes through a DC block capacitor 13a, and reaches the gate of an FET 15 via a transmission line 14a. A short stub 18a composed of a transmission line 16a and a capacitor 17a with one side grounded is connected to the transmission line 14a and the capacitor 13a, and these form an input matching circuit. A gate bias supply terminal 19 is connected to the short stub 18a, and supplies bias to the gate of the FET 15.
Further, the drain of the FET 15 is connected to the DC blocking capacitor 13b via a transmission line 14b, and the drain of the FET 15 outputs a signal to an output terminal 21. A short stub 18b composed of a transmission line 16b and a capacitor 17b with one side grounded is connected to a transmission line 14b and a capacitor 13b, and these form an output matching circuit. A drain bias supply terminal 22 is connected to the short stub 18b, and supplies bias to the drain of the FET 15.
In this amplifier, impedance matching is performed by the transmission lines 14a and 14b and the short stubs 18a and 18b, and the amplifier also functions as a bias circuit. As a result, according to a simulation result (not shown) of small signal characteristics, gain will be maximum and reflective properties will also be minimum near 60 GHz. Accordingly, impedance of input and output is matched in a desired band.
By the way, generally a k factor derived from an S parameter is used as an index of stabilization. In order for the circuit to be stable, a condition of k>1 is necessary. A calculation result of the frequency characteristics of the k factor of an amplifier of FIG. 11 is shown in FIG. 12. According to FIG. 12, the k factor of the amplifier of FIG. 11 is k<1 in the frequency of 2 GHz or less. There is a possibility of being instable in this frequency region such that the circuit oscillates.
Further, as a method to solve such problem of instability in a low frequency region, there is a known method of incorporating a shunt RC circuit composed of a resistor element and a capacitive element in a bias circuit. FIG. 13 is a circuit diagram showing such a bias circuit. A shunt RC circuit 11 is inserted between a short stub 18 which makes a part of the matching circuit, and a bias supply terminal 31. Since a low frequency signal which cannot be grounded by a capacitor of the short stub passes through a large capacitive element of a stable circuit and attenuates by a resistor element, the amplifier is stabilized.
Moreover, in order to solve such problem of instability, in a high frequency amplifying device according to PTL 1, an active element and a matching circuit for the active element are used. That is, a resistive component of input impedance of the active element is made small enough so that a stable index k factor in the single active element will be one or less in a frequency band using the amplifying device. Then, a stability index k factor as the amplifying device is set to be one or more using a loss of the matching circuit.
Further, PTL 2 discloses a technique concerning a spiral inductor that can reduce parasitic resistance between an inductor and a substrate when forming the inductor using a wiring layer on a silicon process. Technique concerning the spiral inductor is disclosed also in PTL 3 and 4.